text:
What's the effect of density altitude on take-off speed and acceleration?
For the EASA PPL(A)-exam I got the following question:

An increased density altitude has the following effect:

Increased take-off speed and decreased acceleration
Decreased take-off speed and decreased acceleration
Increased take-off speed and increased acceleration
None of the above

As this question is ambiguous, I asked the examiner which speed was meant: indicated airspeed IAS or true airspeed TAS. She confirmed that the question was about indicated airspeed IAS.
In that case I would say "4. None of the above". However, during debriefing a fiery discussion amongst the students arose and we didn't reach an agreement about the question. So I'm checking here... [EDIT: btw, I passed the exam :-) ] [EDIT: after the discussion the examiner counted both 1. and 4. as correct! If we're talking about IAS it should definitely be 4. in my opinion; if we're talking about TAS it should be 1. in my opinion. Just checking with you guys.]

During the debrief, were you not told the correct answer? I agree, as worded, it is ambiguous. If you want to test knowledge, you need to have a clear question. They should have stated TAS. That would have been the best way to test knowledge on high density altitude. Due to the ambiguity, the best answer would be 4) None of the above. If they had stated IAS it still would be 4) because IAS does not really change with density altitude.

What Mike Sowsun said x100.

Higher density altitude means less dense air, does it not? If so, then taking off at a density altitude of 10,000 feet compared to zero feet (sea level) will furnish less oxygen to the engine, reducing the horsepower output, and at the same time will require more airspeed to produce the necessary lift for takeoff.
Less horsepower yields less acceleration and at the same time requires greater true airspeed to take-off, so the right answer in that case is #1. If "airspeed" is defined as indicated, or IAS, then the correct answer is #4.

More TAS, same IAS.

"…means less dense air" - and there I was reading it as an "increased-density altitude", i.e. a lower altitude :-)


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Trailing edge flap disadvantages
What does ‘high local surface curvature’ mean with respect to trailing edge flaps?
I’ve been reading up on them, and that came up as one of their disadvantages?

Can you post the complete sentence that doesn't sound good to you?

"High local surface curvature" means greater tendency for airflow separation, which increases drag.
When landing, this can be beneficial to a very "slick" aircraft because it steepens the glide angle and helps control airspeed.
The term "streamlining" refers to smooth gentle curves on aerodynamic surfaces, tending to keep airflow attached.  "Laminar" is best for lowest drag.

Hi Robert, I’m still a little confused about what high local surface curvature still means? Does it have any meaning relating to the geometry? Could I say it means too much curvature please?

@MikeKnows Yes, or a sharp bend or angle.  Moving air becomes turbulent if it cannot follow a curve.  A little like a car going to fast for a turn.  The gentler the turn, the smoother the flow.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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Can someone help me understand this aerofoil?
I came across this morphing aerofoil, but don’t understand how it works please?
Can someone explain also what the components are, like that box in the left?
I looked into that reference, but it’s a complex patent, and it didn’t explain the components

This is a rotorcraft airfoil. Item 206 is the rear wing spar. All the information you need is provided in the actual patent application.
US Patent Application for COMPLIANT STRUCTURE DESIGN FOR VARYING SURFACE CONTOURS Patent Application (Application #20170274976)

Description of the Design Challenges

Designing an adaptive control surface for a rotorcraft poses significant challenges.

Patent History
Publication number: 20170274976
Type: Application
Filed: Jun 13, 2017
Publication Date: Sep 28, 2017
Applicant: FLEXSYS, INC. (Ann Arbor, MI)
Inventors: Joel A. Hetrick (Ann Arbor, MI), Sridhar Kota (Ann Arbor, MI), Gregory F. Ervin (Canton, MI)
Application Number: 15/621,117

FIGS. 18(a), 18(b), and 18(c) are simplified schematic representations of a layered structure arrangement 200 that is provided with web-like structures 202 that are, in this specific illustrative embodiment of the invention, bonded to compliant skin 210, which will be described in greater detail in connection with FIGS. 18(b) and 18(c), below. Referring to FIG. 18(a), layered structure arrangement 200 is shown to be provided with a drive bar 204 that applies a linear force against rear wing spar 206 by operation of an actuator 208. The motion of drive bar 204 is transmitted to a compliant skin 210, the motion of the compliant skin being accommodated by a sliding joint 214 that in some embodiments of the invention may be configured as an elastomer panel (not shown).

Hi Mike,  So does that mean the box is a drive bar? I thought the component behind the box was that?

Yes, (204) is called the drive bar. It is kind of like a movable wing spar that causes the outer skin to extend. The actuator (208) pushes against the fixed rear wing spar (206) which causes the drive bar (204) to move. The drive bar (204) is attached to the outer skin (210). When (204) moves, so does (210).

@MikeSowsun Patent language is impossible obtuse. I don't understand my own patents, and yet I was the inventor! It's a WAG, but I figure that patent language is comprehensible by less than 1% of the population. For instance, "... a layered structure arrangement 200 that is provided with web-like structures 202..." is grammatically incorrect, and yet that's SOP. The upshot is that it would be nice if you could provide a plain-language description of what the patent is actually saying.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Why do expansion fans cause oblique shock waves to curve?
When there is an expansion fan aft of an oblique shock such that its bounding mach waves intersect with the oblique shock, the oblique shock is curved in that region. What is the physical phenomenon which causes this curvature?
Is it due to the static pressure difference between the air on either side of the shock where the expansion fan accelerates the air after the shock and therefore decreases the static pressure?
Or is it due to the fact that for the mach numbers before and after the shock (with the latter being accelerated compared to the case without the expansion fan) to be consistent with the shock angel it has to be more oblique?

Do you have any diagram of the situation you are imagining? I can't find any diagram with a shock and an expansion fan that would show the shock to curve.

Hello Theo, welcome to aviation.stackexchange.com. Very nice first question, we are looking forward to see more of these!

The oblique shock exists to turn the flow.  Its angle (and strength) determines the turning angle.  Along the body (say a 10 deg diamond airfoil), the boundary condition dictates that the shock must turn the flow 10 deg.
The fan exists to turn the flow the other way.  At the crest of our 10 deg diamond airfoil, the fan turns the flow 20 deg away.  (There will be a final 10 deg TE shock to turn the flow back to zero degrees).
Next, imagine a streamline somewhat off the body -- say an inch or a centimeter if you prefer.  That streamline must shock and turn because the 'lower' streamline turned -- it created a new boundary condition.  The streamlines can not cross, so if the 'lower' streamlines turn, so must we.
However, when we get to the turn the other way, only the on-body streamline must turn instantly.  Higher streamlines can turn more gradually -- this is why an expansion fan has finite width -- it is a fan.  The streamlines moving through the fan gradually turn, until at the end of the fan, they have turned the required 20 degrees.
The Mach angle of the leading edge of the fan is such that it will eventually hit the shock.  This more complex area of the flowfield is usually not pictured in undergraduate texts as it is more complex.  Good for you for having this question.

The streamline just above the blue streamline does not need to follow the body -- it only needs to follow the blue streamline.
When the fan reaches the shock, it relieves the need for the shock to turn the flow as much.  Instead of turning 10 degrees, a streamline may only need to turn 9.5 degrees because the fan has turned the next lower streamline.  Then, the next streamline only needs to turn 9 degrees, etc.
A shock that turns the flow less (at the same freestream Mach number) will be a weaker shock, and will have a lower shock angle.  This results in the curved shock.
This kind of 2D compressible flow is a lot of fun.  It relates to not only airfoils, but inlets and nozzles for engines and shock diamond flow in a jet.  Everything that happens is a reaction to the boundary conditions.  Think of what the boundary forces the flow to do and you'll be able to think through very complex flows.

Very nice answer

Thanks for your clear answer answer Rob. Just a follow up question: what's the mach nb after the curved shock? From my understanding there are 2 ways to define that mach nb: 1) The mach number after the shock with the slightly reduced flow turn angle as you discussed found using the shock equations. 2) The mach nb after right after the leading mach wave of the expansion.  I'm asking this question because I'm trying to write some code to compute and plot the curved shocks off of a diamond airfoil and calculating the mach numbers after the shocks using these 2 methods gives different results.

I don't think you can use the pure shock/fan equations / tables for this kind of calculation.  I've usually seen it completed with the method of characteristics (or supersonic linearized theory).  On the one hand, the shock solution to the flow downstream of the slightly weaker shock must be true -- and the inflow condition to the fan will depend on that weakened shock (and so the fan solution must somehow be iterative).  So I think I understand why your answers are different and which is 'right'.  Unfortunately, that won't help you progress through the rest of the flow as the MOC is needed.


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Has this "straight through" airport design ever been contemplated?
Here's an airport, with some planes on the various runways/taxiways and some using the terminal ..
(airliners in green)

Airport/terminal design is really tricky.
There are many spatial problems which are unsolved, resulting in the incredibly inconvenient (for humans/planes) current airport paradigm.
Has an "over" terminal ever been contemplated?

terminal indicated in blue

So, the (whole) airport terminal building is simply up on stilts.
(Obviously, the clearance is high enough for the tallest existing and forseeable airliners.)
At a stroke this utterly solves two problems:

Airliners never again need to reverse.  They just land, drive around anywhere they want, stop under the terminal.  Passengers get on and off.  The airliner drives off again, drives to a runway (perhaps again passing under the terminal - no issue) and flies off.

No craft would ever reverse.
This would be a complete paradigm shift, slashing handling times, costs,  personnel, machinery.

The fundamental spatial problem of airports is solved at a stroke: You can land or take off anywhere, and get to and from, any gate.  (Sure, you have to avoid the few pillars holding up the terminal.)
From the human crowd movement viewpoint, the need for the hell of "separate terminals" is slashed or even eliminated. At worst you'd just have two or three "ranks" of these (rather like giant rows of car parking in a parking lot).

Again this would spectacularly slash the size and infrastructure costs of airport concept design. Often the whole concept of ridiculous people movers, monorails etc would be eliminated or slashed.
Note that the car freeway system would just connect to the end(s) of the terminal building. (Indeed, as usual.)
(But indeed, perhaps the blue structure could indeed include "the road itself". So the blue object represents both car driving lanes and the terminal structure, shopping, etc.)
On that: looking ahead, it's possible there could finally be a drive to your actual gate concept (if check-in / baggage handling / border functions could be rationalized), which would be an total paradigm shift in air travel.
The only technical difficulty is AeroTech or whoever would have to make a jetway that comes "down"...

Note that it's obviously trivial to make huge civil structures "elevated",

(Indeed a civil structure for foot traffic, shops, etc is (I believe) a lighter load than a super-freeway or bridge,

)
Ideally the supporting piers would be infrequent, allowing for totally free and easy movement of airliners, but even if the columns were between each "bay" the major paradigm shifts are achieved.
This airport paradigm would seem to be staggeringly more efficient than the current paradigm.  No reversing ever!  Free driving anywhere for all craft over the entire area.  Wasted space slashed.
Has this ever been contemplated?

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On my computer, the top images shows just airplanes and tarmac, but no terminals.  Should it show terminals, like the other images do?

I can't see that the current airport paradigm is "incredibly inconvenient", at least absent the security theater.  It would seem that this design adds a number of new inconveniences, like getting wheelchairs or people with limited mobility up & down those jetways.  Then too, it'd seem pretty easy to clip a wing on one of those supporting pillars.

"Drive to your actual gate" is a matter of fact at Berlin Tegel Airport (Terminal A) http://cdn4.spiegel.de/images/image-414893-breitwandaufmacher-bdkt-414893.jpg - the hexagonal building with dark red jetways and planes parked around it, and cars "inside" the building is what I'm talking about. The taxi driver always asks you for the gate number if your plane departs from Terminal A and he/she will actually drop you at your gate.

Fantastic info, @ElmoVanKielmo  .. ah wait, is that traditional Tegel or the new airport???

Traditional Tegel. The new airport is not going to be opened anytime soon.

Ahhh!  OMG I love Tegel, i thought it was gone!  Yes!  Of course!  it's a "drive to terminal" airport! thanks @ElmoVanKielmo

I hereby dub this the "Forecourt" design, or Service Station, Gas Station, Petrol Station, or whatever you call the place where automobiles fill up with liquid fuel.

Have you considered putting the access bridge below-ground rather than overhead?  Airports tend to be windy with fast-moving objects.  Tunnels below the apron would be much more protected, and able to serve taller planes in the future without having to raise the walkway.

@Criggie , indeed yes, "GMTA".  For me that is the obvious and perfect way to build airports.  The entire thing, seen from above, should just be "an enormous open area".  (I guess, apart from the control tower.)   Massive parking structures and everything else would be entirely underground.  I agree totally!

do see @747 's answer!

Yes. An architect named Miklos Deri seems to have contemplated something similar: Drive Through Airport – Inversion of Procedures. The design is based around smaller aircraft (which reduces some of the height and size problems) and stacks aircraft together in lines to address the space efficiency issues.
Nobody has built such an airport (it's a concept, not created for any particular project), and there are practical problems associated with moving aircraft multiple times during a turnaround.

Ah, ............ The Answer.  Nice, good one!

Damn it, that guy's renderings are much better than mine. It's funny he went with the "reduce CO2" angle to try to get funding  :)

So what happens if the first airplane in the row (or any one in front of the plane you're on) goes on a delay? Everyone behind it waits?

@SportsRacer That would be another example of why nobody would build this.

Though I've never heard of a terminal design which allows the airplanes to pass under the passenger areas, the same problem could be solved by having the airplanes pass over the passenger area. There exist several underground airports in the world, most notably the Denver International Airport.
Though all civilian pedestrian traffic in Denver is routed above ground just like any other airport, it seems that the underground networks could serve the same purpose without many major structures above-ground. This idea could be extended into an airport with a design with advantages similar to that in the OP question, but much easier and cheaper to build and maintain.

How is the Denver airport underground? It has many underground tunnels and structures, like a number of airports do, but aside from conspiracy theories, aircraft are boarded other airports from above ground.

Underground is a whole lot more sensible than raising everything up. The only problem with that is user experience. But a good architect can deal with that.

@ZachLipton: I mention that civilians use the above-ground facilities. However, Denver sets precedent for having facilities on a different level than the ground.

Well, CDG T1 uses a similar concept, it has tunnels, and AFAIK, some of the planes don't push back but rather continue towards the main building and then above the underpasses.

It seems to be that CDGT1 is indeed the "straight through" concept (but underground rather than elevated).  I had no idea about this, good one.

There are several parts to your "design" that may have been implemented, though not exactly in the same way you have them in your post.

Building terminal structures with planes passing underneath them: one example is the bridge linking Gatwick's North Terminal to Pier 6. It allows very large planes to taxi underneath, so it is very, very high, which makes it very, very impractical (you have to go up and down very very long escalators), especially considering many passenger carry hand luggage, etc.
The alternative in most cases is to have passengers (and luggage, and many other things) run under the taxiways rather than above them. This is what is done for "remote terminals" in most airports that have them. Here are a few examples (but the list is definitely much much longer):

Geneva airport:
Paris Charles-de-Gaulle Terminal 1:
London Heathrow Terminal 5:
Hong Kong airport:

Note that instead of having one access from the overhead/underground structure to each plane, there are usually satellites / peers that will allow access to several planes at once.
Depending on the actual layout and other parameters, passengers are carried between the building using moving walkways (CDG T1 and GVA in the examples above) or an automated people mover (LHR T5 and HKG in the examples above). Depending on the airport design, you may have to go down and up very long escalators as well, but given how frequent that design is over the bridge design, it must have advantages (more future-proof, larger capacity...).
Then there is the issue of having planes that can depart from the gate withough needing a push back. They are parked more or less parallel to the satellite, and once the jetway has been moved, they can just taxi forward.
You can see the first two examples above (Geneva and CDG T1) apply this concept. It is not necessary very space-efficient, and is not as extreme as the design in your post, but it is definitely in use (and has been for decades).

My God, Gatwick has an example of this, I had no idea!   Gatwick!  Priceless info, thanks @jcaron  (I wonder if or how they overcame the "falling small pieces of material" issue, mentioned by a commentor.

If you are looking for structures going over taxiways, then Denver has one (though much lower, so limited to narrow-body aircraft passing beneath), and Phoenix has an APM bridge.

Geneva looks to be the closest answer to the OP - a plane can taxi to the parking spot, then pull away on its own when it's done with all servicing. While the round structure doesn't appear to be the most space efficient, it does seem to allow for the greatest flexibility of aircraft sizes.

"many passenger carry hand luggage" - and at least a few use wheelchairs or other mobility aids, and they also need to be adequately provided for.

Similar elements in airport design have been considered, but not this exact design.

This is a rendering of a real-life plan for a renovated LaGuardia terminal. As you can see, it's not a single wide strip. Space is expensive and the design in the question results in an extremely wide airport. The picture above, if you align the planes with the taxiway/runway, achieves the same effect more space-efficiently. You can load up several planes standing behind one another, there's no need to build everything up in width.
It's actually not trivial to make large civil structures elevated. The examples in your pictures look impressive precisely because it's not trivial. Supporting spans long enough to fit large airliners, with clearances, takes some effort.
Realistically, an airport terminal like what you're proposing would have to use bridge construction methods, with the deck replaced by the terminal. Whether bridges are easier to build than tunnels, as used in Denver, varies by site, but it's fairly realistic.
Aviation is a cost-sensitive business; there's also a lot more to an airport than the passengers see that you'd have to accommodate. Bridge or tunnel style construction costs considerably more than flat boxes, so it won't become the new normal. But in well-off cities willing to spring for the airport's construction, it can be done.

Fantastic information, Therac.  Indeed, it would appear this is the closest-to-happening version of the concept under discussion - good one.

With the exception of the pedestrian bridge over the taxiway, I'm not sure how this fits with the "straight through" paradigm that @Fattie was asking about. I'll give it a +1, though for the "It's actually not trivial to make large civil structures elevated" comment - building even a pedestrian supporting structure high enough to clear an A380 and wide enough to clear the 80m box a plane parks in, plus some additional clearance for non-precise taxiing between supporting pillars is non-cheap.

Airports including Gatwick and Denver already have pedestrian bridges over taxiways. As FreeMan noted, I'm not sure how this fits with the question.

@FreeMan I should have elaborated more. The design element depicted accomplishes the purpose of the proposed design. You can have a straight-through, no-reversing movement of aircraft. But it's done with less footprint. Just line the planes up, 2xN, extend the taxiway up to a runway, or use that and the runways, and you have the result. It's simply unnecessary to reduce the queue to 1-plane-long, because gate time is on the critical path.

@Therac - I'm sorry, I'm still not following you. If the planes were "Parallel Parked" along the terminal such that a maximum nose-wheel steering angle would get them out without having to be pushed back, I could see that, but based on the image posted, I'm not seeing that happening here. I've not read your link, so maybe there's info there that is missing from the context of your answer that would clarify what I'm missing.

@FreeMan Consider that these are not real planes, just models on a mesh. You could easily turn the planes and the bridges to parallel park them. IOW, this design supports what amounts to straight-through operation.

@OK. I suppose, with long enough jetways, though, most (well, many) airports could support that mode of operation as they currently exist. It's simply a matter of having enough space for wing tips not to touch, and there you go.

The main benefit you propose for the elevated terminal is that planes wouldn't have to push back from a gate, but planes pushing back really isn't a big enough problem that you'd want to build these massive amounts of infrastructure to solve it.
Returning to the original "air field" approach of having a large area where planes can move in whatever direction is most appropriate doesn't seem workable. Other answers have pointed out that ATC would become very difficult. Planes would no longer be constrained to taxiways and runways so how would you even have the conversation where a pilot says they'd like to take off, please, and the controllers direct them which way to go?  In addition, you'd need to pave a truly enormous amount of land to airport runway strength, which would be phenomenally expensive. And the airport would need to be twice as long to allow planes to land, go to the terminal and take off again without turning round. Even more expensive.
It's not clear how the "straight through" concept would work with varying wind directions.  What if wind conditions today mean that planes want to take off at right-angles to the terminal building?  Also, even at airports where all the runways point in the same direction, it's not uncommon to change the direction of operation as the wind changes.  With a "straight-through" airport, you'd have a bunch of planes pointing in the wrong direction.
The "drive to the gate" concept isn't actually new so we already know it doesn't work.  Kansas City International Airport (MCI) was built that way in the early 1970s.  Unfortunately, that was just before airport security screening became a thing. "Drive to the gate" fundamentally requires separate security screening at every gate, or shared between small groups of gates, which is extremely inefficient.  As soon as you try to share security between larger groups of gates, you lose the "drive to gate" concept because (a) the security area is used by many people, so there's not enough parking space nearby; (b) it's shared between a lot of gates, so there's not enough space for all those gates to be nearby; so you end up driving to somewhere quite far from security which is, itself, quite far from the gate.  Basically, you're back to the traditional airport design. When MCI was built, TWA insisted on the "drive to gate" format, it was found to be awful within a couple of years, TWA insisted that the airport be rebuilt, the city said "Sorry, dudes, but we just spent an absolute fortune building this thing to your design; we can't afford to do it all again" and TWA said, "Screw you, we're moving to St Louis."

Great example about MCI ...

DFW has a similar design to MCI, and it's one of the busiest airports in the world, so it's not fatal.  However, the "thin" design is clearly not as efficient as piers (see DEN, ATL or PHX-T4) when used as a hub (mostly connections), which wasn't really a thing yet back when DFW was designed.

@StephenS DFW looks more like "drive to a pretty small terminal" than the literal "drive to gate" of MCI.

@DavidRicherby At DFW A/B/C/E, the parking slots are labeled by what gate you're across from because, before security checkpoints were retrofitted, you'd park at your gate number and walk straight through the (very thin and open) terminal to your gate.  DFW D (much newer) was designed from the start to funnel everyone through one big security checkpoint; there are no gate numbers in its parking garage.

@StephenS OK -- thanks for the explanation!

Parts of MCI are still a narrow miserable widely-disliked mess thanks to these design decisions and the retrofits to add centralized security.

@ZachLipton Interesting that a score of 744/1000 puts MCI in 26th place in a class of 33 when the top score was only 794. To me, that says that people basically dislike all airports, but dislike MCI slightly more.

To answer the question: No, this concept has never been considered, at least not seriously. The reason for this is

You can land or take off anywhere, and get to and from, any gate.

This would create a nightmare for anyone planning arrivals and departures, and is sure to result in utter chaos (not to mention crashing planes).
However, as some has noted parts of this concept has been used. Overhanging gates, walkways, terminals and so on. What you have described in your suggestion is basically (to my understanding) piers as used on airports today, although with a different shape than we are currently used to.

(...)  the need for the hell of "separate terminals" is slashed or even eliminated. At worst you'd just have two or three "ranks" of these (rather like giant rows of car parking in a parking lot).

These are piers. An airport holds a lot of people during peak hours. Not everyone is on their way to an aircraft ready to board, not everyone is collecting their luggage and driving off. A lot of people are early, transiting between flights, delayed or there for other reasons.
Having a large terminal helps with the crowding, as the people can be spread out over a larger area. Piers can then be built with a smaller capacity in mind, as you keep the number of shops and dining areas to a minimum to avoid attracting people who have to wait for whatever reason.

They just land, drive around anywhere they want, stop under the terminal. Passengers get on and off

Again, this would be chaotic. Passengers need to know where the plane will arrive. The ground controllers has to know so they can coordinate the planes movement with any other plane on the ground, ground services (food, fuel, cleaning) needs to plan how much to deliver and where, plus the fact that different aircraft require different infrastructure on the ground.
An A380 carries more people than an E-170. Allowing the pilots to just select a gate might result in an A380 arriving only to find that their gates are occupied by smaller aircraft. If they are at all able to get into a smaller gate and unload, the areas around that gate might not have room for the embarking and disembarking passengers, leading to congestion and delays.

This airport paradigm would seem to be staggeringly more efficient than the current paradigm. No reversing ever! Free driving anywhere for all craft over the entire area. Wasted space slashed.

There are many factors in play when designing an airport. Cost and complexity are two, but regardless of how efficient you might create the airport you are still faced with the fact that you need to schedule movements. People will always be dumb, slow, confused and unfamiliar with your airport and airplanes will always be delayed or suffer breakdowns, and your facilities need to account for this. Your concept might work if everything else was working perfectly/to schedule, but I fear that once a disruption occur it would propagate and gradually cause more and more problems.

I'm not sure that OP intended for the pilot to pick a gate as you  & I do a parking spot at a car-park. However, having one, miles long terminal would be an absolute nightmare for the passengers arriving from the parking lot at one end and needing to board an aircraft at the other end. It would be bad enough if the car parking access were in the middle of the terminal. Not to mention trying to make a connection from an arrival at one end and a departure near the other end.

Amsterdam's Schiphol Airport sort of uses this property, but the planes don't end up underneath the terminal. Rather, the ground underneath the terminal is used for vehicle traffic and whatnot. When you get off the plane, you get picked up by a bus and brought to an area where there is an escalator/elevator to bring you up to the terminal level.
I reckon airports don't want to use your design because a chunk of building material could cause expensive damage to an aircraft, even if the chunk was the size of a grape.

that's a good point about falling material - good one

The reverse is also true - a damaged aircraft could easily cause flames to damage the building above.


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Given that the cabin is pressurized, I assumed that means sealed, how basically do airliners take in air (ie, for ongoing breathing, etc) when high?
Given that the cabin is pressurized, I assumed that means sealed, how basically do airliners take in air (ie, for ongoing breathing, etc) when high?
What is the valve?-like mechanism that does this, or .. ?
Or are they not really sealed that tight, and they are pressurized by just continuously pumping in a bit of air?
(I think the cargo area even is pressurized on some/all? airliners these days, is this part of the equation?)

See How do big planes like 737 manage fresh air for passengers during flight? and the answers on this page for full schematics.

The jet engine compressor section produces an ample quantity of high pressure air.  When we divert some of that air (rather than letting it continue to the combustor and turbine), we call it 'bleed air'.
Some bleed air is used in the engine itself -- primarially for cooling the turbine.
At startup, bleed air from one engine is used to start the other engine.  An APU (small jet engine in the tail, auxiliary power unit) produces compressed air to start the first engine.
In flight, bleed air is used for the ECS system -- environmental control system.

It is clean and fresh.  If it smells bad when you're on the ground it is because of the exhaust of another nearby aircraft.  Once in-flight, this is not an issue.  After going through the compressor, it is hot, so it will pass through a heat exchanger to cool it down.  It will also go through some sort of evaporator / dryer before the heat exchanger to remove moisture from the air -- this prevents the moisture from freezing and causing blockage.

Additionally, there is an outflow valve, which intentionally continuously lets air out of the aircraft to regulate the pressure to the desired pressure and to allow a continues flow of fresh bleed air in for breathing etc.

They are really not sealed that tight. There can be leaky areas, and there is one or more outflow valves that intentionally leaks air.
To get new air, a valve is not enough. The pressure outside is lower, so an open valve lets air out rather than bringing in new air. Instead, a compressor of some kind is used. Turbine engines compress air anyway as part of their operation, so many larger airplanes redirect air from the compressor of the engine to use for this purpose. But there are also airplanes with a dedicated air compressor for cabin pressurization.

Cabins are pressurised in the sense that they are designed to keep the pressure inside them to a value higher enough to let an averaged human body breathe without difficulty.
Anyway they are not 100% airtight for one simple reason: let's say that on ground the pressure is 100 (we don't care about the units of measurement right now). At the standard altitude at which a jetliner flies, the pressure is 20. 20 is a way too small of a value for a human body to be able to breath without an oxygen mask therefore we have to isolate the fuselage and let the pressure inside stay higher than 20. Ideally we could perfectly isolate the fuselage and keep the pressure at 100 but this would be critical from a structural point of view: the fuselage would be "pushed outside" by the inside pressure of 100 and "pushed inside" by the outside atmospheric pressures of 20 i.e. the fuselage should support a net pressure of 100-20=80. This would be totally possible, but the structure would be so strong and heavy that the airplane could simply not fly.
As a compromise we actually let air flow out when climbing so that, when the cruise altitude has been reached, the pressure inside the cabin is 60: this value is lower than the 100 on ground but it's still breathable and, most importantly, gives a net pressure acting on the fuselage of 60-20=40 i.e. half of before. This net pressure allows the fuselage to be built light enough to be safely brought in the sky.
Obviously now the problem arises that we cannot simply open the window to change air due to the difference in inside vs. outside pressure: a system working with pressurised air has to be used as nicely explained by @RobMcDonald in his answer.


text:
In 1997, what would the likely Paris–Tokyo route be? Generally, where to gain information on historic routes?
Today you would likely via Dubai, or perhaps take one of the long flights say Zurich -> NRT.
If I'm not mistaken Dubai came to it's current position in early 2000s, so in the 90s it was not the typical international hub it is today. And the longer Europe -> Asia legs were not common.
What would have been a typical route in the middle nineties?
More generally, is there an "expedia of the past"?
As a child (70s) I know all the typical routes, say, UK - Australasia in that era, but they have all changed drastically. Is there any way to find or know the typical routes of past decades? (Other than asking now-old airline staff!)

(there are a number of similar excellent questions on the site, eg https://aviation.stackexchange.com/questions/53584/what-was-the-flight-route-from-new-york-to-paris-that-air-france-took-in-1955 )

It's a shame this knowledge is being lost forever.

There are direct flights …

@Antzi in 1997 ??

I don’t know. I meant today.

I see. Yes, I've flown that route many times, today.  Notice the question at hand (notice first two words) is about 1997.  Cheers.

The first sentence of the question seems to imply that a direct flight is not usual, mentioning the Zurich (?) Narita

hi @Antzi  pls read the title. the question is about in 1997.  thanks

I think what you want for this question,and other similar ones, are old copies of "The ABC World Airways Guide" .
This image, from a 1991 copy, gives you some sense of what was contained within it https://1.bp.blogspot.com/-1E9sPe3jKGQ/Ur7xyNPWerI/AAAAAAAAMTI/9T9ARGBdCDE/s1600/PK+pia+pakistan+1991+1.jpg .
It showed you the points at which a flight landed but it didn't indicate the routes between those points ... I think it's the former rather than the latter you want ?
From googling around I can see large libraries which hold copies of these.
UPDATE: This blog post https://airline-memorabilia.blogspot.com/2013/12/abc-world-airways-guide-1991.html contains a number of other images from a 1991 copy of the guide.
UPDATE2: This is also an amazing, to me at least, resource http://www.timetableimages.com/ttimages/complete/complete.htm .

Glaucon, this is an incredibly useful answer.  Thank you!  (I'm a bit surprised there is not more interest in this question!)  Again thanks!!!

Fattie - It's certainly stuff that interests me but I guess it's not for everyone ;-) I remember thumbing through ABC World Airways Guide. Apart from the timetables there were lots of fascinating footnotes about obscure places ... rather geeky but then I am !

Heh - such is life


text:
Is the "Boom" project vaporware or really going to happen?
Is this https://boomsupersonic.com viable or more vaporware?
Any solid information?

It's "not my site" but it is incredibly naive to mark this as "opinion based".  A wealth of factual information on contracts and finances could be presented here.

It might help to add more to your question explaining the details you're looking for.

"Is it really going to happen?" cannot be answered here, unless one of the project managers drops by. If you were to ask if it is feasible given x, y, z conditions, then we can have a go at it. Your wording is a bit too much in the middle.

I'm afraid that just doesn't make sense, @AEhere .  Say someone in the air finance industry on here stated "As a matter of fact, they have N orders from X airline who have already paid M million euros on D date".  Surely, you would agree that is extremely firm and factual information towards the subject???

@Fattie it's evidence that an airline was willing to throw money at the project, no more or less. I'll echo what others have said so far about this question being opinion-based in its current format since there's no way to confidently say that anything will come of it.

Technologically, it's possible -- though I question the "go twice as far" since supersonic aircraft typically consume much more fuel than their subsonic counterparts.
Economically, I doubt it'll be a success: unless you have technology that somehow limits or muffles the sonic shock wave footprint on the ground, and can prove it to regulators, your aircraft will be banned from supersonic flight over populated areas.  Operating and maintenance costs will push ticket prices, er, into the stratosphere, just as they did with Concorde -- and with the relatively small fuselage diameter that goes with supercruise, passengers who aren't dwarves will think twice about offering repeat business.
There may be a market for a supersonic bizjet -- and the BOOM illustration suggests this might be their target market (I don't see a lot of windows).  They'll still be limited top subsonic flight over land, however, which narrows the market to those who need to cross an ocean (that's not too wide) very quickly.

You know, i just naturally assumed they were using the anti-boom technology which has been around for ages now. (Eg, this demo is actually being welded up at the moment, I believe  https://www.newscientist.com/article/2165683-nasa-is-trying-to-build-a-supersonic-aircraft-without-the-boom/ )

While this answer is admirable related opinion, thanks, the question is asking for actual industry knowledge of the company / project .. hopefully someone has that

@Fattie The NASA boom-suppression test vehicle is, as far as I'm aware, the first attempt to actually flight test the technology.  What appears to work in CFD software or in a supersonic wind tunnel may not do what it needs to do in the air.  And jumping the gun on NASA seems like a pretty high risk level...

Zeiss - ah that is the first attempt.  Fascinating, thanks for that

And I might add that BOOM doesn't appear to incorporate any of the NASA tester's special feature (like the extremely slender nose -- requiring the pilot fly via camera and monitor screen -- and shockwave trapping canards).  BOOM looks like a scaled down Concorde, which is 1960s (or, charitably, 1970s) tech.

The second section about the Concorde is incorrect and a common misconception it was quite profitable and did not lose money, it was actually one of BA's best earning aircraft. One of the former pilots covers everything in this podcast (worth a listen). If anything it was becoming expensive at the end of its life time as small batch parts suppliers closed up shops and the plane suffered from the higher wear of supersonic flight.

@Dave And yet the routes were never expanded -- for instance, a Mumbai to Sydney or Mumbai to Kuala Lumpur would have been within Concorde's range, and over water (almost entirely, with a little route management) -- and has money attached -- but no Concorde on that route.  If it was profitable, it should have gone more places than just the one route.  Tokyo to Anchorage to San Francisco is another route where it could have "beat" conventional airliners -- but it never flew anywhere but the Atlantic crossing.

@ZeissIkon you should listen to the podcast... A solid chunk of the Concorde's flying was actually charter operations to lots of places. It also flew other schedule routes aside from the NY London trip

@Zeiss Ikon a reason for atlantic crossings only routing (excluding charters mentioned by Dave) might very well be that it was the most profitable option. And then there's the fact that when something broke down, "homebase" was very close: imagine a technical at Sydney, with spares at good ol' Europe... the more expensive the plane, the more it must fly to keep it economical.

I thought the whole point of these smaller supersonic biz jets was that they were smaller and lighter than Concorde, which would reduce their boom to the point where they would be allowed to fly over land?

@RobinBennett I don't know that that's been demonstrated.  Are any of them flying yet?

No, none of them have flown or demonstrated a reduced boom, let alone actually receiving permission to fly supersonic over land - but I think they are reasonably confident of all three, or they wouldn't be investing in the project.

@RobinBennett Flying, no problem (modern software is pretty good).  Reduced boom, fairly likely, due to smaller size/weight as you note.  Permission to overfly populated areas above Mach 1?  Unlikely in today's environment without near-zero footprint demonstrated.


text:
How long can an aerobatic (or any) plane fly "on its side"?
This recent question Can a plane bank without turning? asked if a plane can sustain some amount of roll in straight flight, sure they can.
Consider an aerobatic plane (perhaps a Pitts?).
As far as I'm aware it's certainly seems possible for them to fly "on their side" ... roll of 90° or -90° ... at least for short times.
I guess, in a "four point roll" you are on your side (90° or -90°) for at least a short time.
In fact - Pitts owners or others! - how long can you actually do that for?
There would seem to be no (upwards) lift at all?
Are there videos of someone flying on their "side" for miles?
Or in fact is it just an artifact .. you can only actually do it for seconds and really just momentum is keeping you seemingly from losing inches of altitude?
How long can you fly on your side for?
What's the deal on this?
If the answer is "only for a tiny amount of time" in fact, can ALL aircraft do that, or is it really only possible for "aerobatic" aircraft, and if so what's the enabling difference?

As a non-pilot (hence comment, not answer), not only can you fly at 90° from horizontal, you can do so as long as you can get fuel to the engine(s). Deflecting the aileron toward the "up" side will cause the plane to adopt a nose-high attitude that gets some of the thrust working against gravity. It also puts the direct airflow against the "down" side of the fuselage which will provide some amount of lift. With enough power directed at the right angle, the engine will overcome the loss of lift from the wings. YMMV based on power to weight ratios.

The time depends on the fuel system. Normally, aerobatic airplanes have a small tank which supplies fuel in all orientations, but the rest of the tanks only works in upright position. So it is the content of that small aerobatic tank which might be as little as fuel for five minutes. With properly modified main tanks, this might change to hours.

I remember watching a replica Gee Bee Sportster doing knife-edge climbs. I'm sure it could sustain knife-edge flight as long as its fuel system allows.

ah thanks for the dupe link !

@Pondlife - actually the linked QA does not at all answer the question (unfortunately).  (1) it has an irrelevant article about radio planes.  (2) there's a link to a video (about what??) which does not work  (3) the only answer that actually addresses the question says "Regular airplanes, no. .. No aircraft that I know of can produce enough lift with its component tilted 90°." with no other info or sources  :O  :O

Can anyone actually tell me if a typical (say) PItts Special can fly at 90° and for how long?

@FreeMan , I completely and obviously understand that, thanks  :) It would be as if I asked "what's the world's longest flight" and you said "Anything is possible with fuel".

@FredLarson - there's no such thing as a knife-edge climb.  if you start on knife-edge and then turn up, it's no different at all than if you started in any other orientation and then pointed up

@PeterKämpf thanks for the comment.  So I roll 90° left.  I think what you're saying is the craft then has to yaw (that would be "upwards", now) say, for example, 20°, and it can then proceed in that attitude.  That seems like "cheating", the long axis of the craft is not parallel to the ground.

@Fattie: As I recall, he started doing a level knife-edge pass, then kicked a bit more top rudder and started gaining altitude. What would you call that other than a knife-edge climb?

heh @FredLarson i get what ytou're saying, and it's incredible he was able to transition from a knife-edge ("merely level") and then turn "right" and be in a straight-up climb.  However consider, in the "end" when in a straight up climb (also amazing) that is "just" a straight up climb!  no different (no more power needed) than "normally" going in to a stright up climb!


text:
Why are FDRs and CVRs still two separate physical devices?
Given the histories of flight data recorders and cockpit voice recorders, I somewhat understand their origins as two separate physical devices. Given their separate tracks of development, regulatory requirements, and storage mechanisms, it stands to reason that they historically existed as two separate "things".
However, I'm under the impression that the vast majority of the commercial airliners operating today are utilizing solid state memory to store both flight data and cockpit voice recordings. Both devices share many similar requirements in regards to being capable of withstanding extreme heat, g-forces, impact resistance, submersion in water, etc. Both devices are equipped with underwater locator beacons. Even the bright orange color of both recorders are the same!
The Wikipedia article on flight recorders briefly mentions that "the FDR and CVR can be manufactured in one fireproof, shock proof, and waterproof container as a combined digital Cockpit Voice and Data Recorder (CVDR)". However, whenever I read about modern aviation accident investigations--1990s, 2000s, 2010s--investigators are always searching for and analyzing the two separate black boxes. I don't think I've ever heard of an accident that involved a single combined black box, so their usage still doesn't appear to be very widespread.
Why not? Since the only thing I can see that distinguishes a modern CVR from a modern FDR is the type of data being stored, why didn't the integration of these separate devices become the norm decades ago? The best explanation I can come up with is some sort of notion of redundancy, but this doesn't make sense if they're storing two separate sets of data.
Even if a single combined recorder was deemed to be too risky, how hard could it be to just record both sets of data on two identical recorders? At least that way, there's true redundancy being maintained. The fact that "flight data" and "cockpit voice" data are still being stored on two separate physical devices seems rather absurd in this day and age.
It seems about as logical as buying and maintaining two separate external hard drives--one for my music collection, one for my movie collection. It'd be closer to life in the early 90s, when I had a collection of VHS tapes and a collection of cassettes. But now that everything is digital, it makes sense (logically and financially) to just use one device to store both collections. Why isn't it the same with modern-day FDRs and CVRs?

Welcome to Aviation.SE! Excellently done first question, well thought out and written. Shows your research.

Well presented facts and analogies. This question was briefly asked and answered in past, but your version is detailed.

While this is a very well written question, this is also a duplicate of Why don't both black boxes hold the same data?.  I hope that you stick around and continue to ask such high quality questions, however please search the site first to see if it hasn't already been answered.  :)

@Lnafziger This user last logged in about 3.5 years ago. :)

@reirab Haha, I guess he won't be sticking around then. Hopefully he comes back!!

You probably wouldn't believe the effort needed to certify anything that flies on an aircraft. This is a multi-year process, especially if it is so much security-related as the CVR and FDR. And it needs to be done with all national authorities of your intended customers.
This is a case of good-enough. The advantage from combining both is real, but so small, that nobody so far thought it worthwhile to go through the certification process. And regarding redundancy, transmitting the recorded data to some ground station would be even better than recording twice on the same aircraft. If that would have been installed in MH370, most of the search effort could have been saved.
I expect something to happen with the transmitting of data, and then both devices could conceivably merged into one.

It's odd to call CVR and FDR 'security' related, since they don't really protect passengers from accidents.

@florisla They do protect passengers. Not of the affected flight, but of all future flights if a serious flaw is found.

Also, if one is damaged (or can't be found) and can't be read, chances are good the other may still be able to yield data. Combine both into a single device and you lose that. So it adds a bit of redundancy.

Actually, it's happening already -- transport helicopters have used a combined CVFDR for ages now, and newer generations of CVDRs have made inroads in VLJ/light bizjet apps where previously, there may have been a CVR but no FDR or no recorders whatsoever.   The Embraer Phenom 500 that went down in Gaithersburg, MD not long ago had a combined CVDR that was used by the investigators as part of the reconstruction of the crash, for instance.

@jwenting Yes, redundancy is important -  that's why I would expect they record all signals in one type of device, and use two of them.

@VolkerSiegel That'd lead to another problem: what if there's a design flaw in the device causing it to not record anything? To counter that you'd need 2 different devices designed to do the same job, or ideally 3. Which is how redundant systems are often designed, 3 different things designed to do the same thing, watching each other.

Many new aircraft, starting with the Boeing 787, are equipped with enhanced airborne flight recorders (EAFR). EAFRs are designed to the ARINC 767 architecture (described here). They combine the functions of the legacy FDRs and CVRs.
The EAFR is designed to be compatible with aircraft using the ARINC 664 network for their avionics. As a result they are capable of recording a very large number of parameters. They are not typically retrofitted to older aircraft due to the complexity of routing data from the large number of independent data buses.
For maximum survivability, each aircraft is equipped with two EAFRs located in different parts of the airframe. Both EAFRs record the same flight data and cockpit audio.

The Cockpit Voice Recorder and Flight Data Recorder are the same unit on many modern aircraft and are widely available at vendors such as this resale outlet and direct from vendors like Universal.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Is a releasable locator beacon (ELT) required for aircraft carrying passengers over a water area?
Is an Emergency locator transmitter - ELT required on any type of aircraft carrying passengers over water? Does it need to be fitted with an hydrostatic release unit so that it doesn't sink with the aircraft?
If they don't carry a releasable ELT, how are aircraft planned to be located after an accident over the ocean?
References to regulatory documentation definitely a plus (e.g. for the US, or for island countries like Australia or New Zealand).

related: http://aviation.stackexchange.com/questions/2153/do-flight-recorders-float-in-water

14 CFR 91.207 for FAA, ELTs are required to be carried, but they do not have to be hydrostatically released since this would require external mounting. ELTs have to operate for 48 hours. In the event of a water "landing", many FDR's have a sonar "ping" for location purposes, but I don't think the ELT is required to operate under water.

I know you are asking about requirements, but Honeywell does make a water activated ELT which looks to be a "portable" unit. It is TSO'd but it doesn't look like something you mount, just carry on board. It doesn't say anything about hydrostatic release. All the other ELT's are G-Force activated.

@mins I know, I was just mentioning that FDR's have the ability to operate under water, while I don't believe that there is a requirement for ELT's to operate under water, since the radio waves don't work very well under water. Its also important to note that Helicopters are not required to carry ELT's for Part 135 operations, even carrying passengers... Read 14 CFR 91.207 closely, it says airplane, not aircraft...

@RonBeyer: Agreeing with your last comment. Does 14 CFR 91.207.f.2 "Aircraft while engaged in scheduled flights by scheduled air carriers" prevent the requirement to apply to airlines?

@mins Yes, that is how I would read that, although it does not prevent an airline from carrying it, it does not require them to. Oddly enough it would require them to be carrying one during a ferry flight since section (e) would not apply, if it were a repositioning flight that is.

@RonBeyer: Yes this is strange, as an aircraft used for a charter flight (not a scheduled flight) would need to have an ELT.

@mins I guess it depends, looking at it again, f.11 could apply to remove the requirement: On and after January 1, 2004, aircraft with a maximum payload capacity of more than 18,000 pounds when used in air transportation.

@RonBeyer: More confusing: "An ELT is required on all U.S.-registered civil airplanes, per 14 Code of Federal Regulations 91.207", and BTW, how serious is mounting an ELT this way :(

So why would they specify in paragraph (a) that it applies to part 121 then turn around and specifically exempt them in paragraph (f)? I don't get it.

Is an ELT required on any type of aircraft carrying passengers over water?

Generally, commercial transport aircraft are not required to carry an ELT.

how are aircraft planned to be located after an accident over the ocean?

I believe pilots are expected to use radio to inform controllers of any emergency.
If ATC are unable to contact an aircraft for an extended time, they instigate S&R based on the last known location of the aircraft.
The FDR and CVR are required to include an underwater location beacon (ULB), though this is only short range and has limited duration.
It is expected that, sometime in the future, commercial transport aircraft will be required to have automated systems that report their position frequently (perhaps every 15 minutes)

References:

IATA - Aircraft Tracking Task Force (ATTF)
ATTF FAQ
ICAO Normal Aircraft Tracking Implementation Initiative

authors: Роман Деньгин

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How is the correct air speed for fuel combustion obtained at the inlet of the combustor?
Many jet aircraft travel at speed of about Mach .8 which at an altitude of 11 km represents an airspeed of about 300 kt or 150 m/s.

If fuel was injected into air at this speed, the mix would just leave the engine before being burnt, combustion would occur behind the aircraft. From different readings, I believe the correct airspeed in the combustor must be around 10 or 20 m/s to maintain the combustion within the combustor.
In the compressor section, air from the intake is forced to rush into a convergent annular tunnel to compress it. Intuitively, this design won't slow air, or not slow it enough.

General Electric J85-GE-17A, source: Wikipedia

How engineers are able to slow air without decompressing it?

Engines aren't drag-less, meaning the air doesn't pass through them unimpeded. The compressor section slows the air down quite a bit. In supersonic aircraft the intake design also plays a large role as the airflow has to be slowed to subsonic before it enters the compressor stage.

In general, aircraft jet engines have diffuser section(s), that reduce the velocity of the incoming air before it enters the combustion chamber.

Sample velocity profile in jet engine; image from Fundamentals of Gas turbine engines
In some cases, the the diffusers are before compressors or in the stages itself- but the end effect is reduction in velocity. Even in this reduced speed, combustion is not feasible as the the speed of burning kerosene at normal fuel-air ratios still lower; hence any fuel lit even in the pre-
diffused air stream also would be blown away.
In order to overcome this, a region of low axial velocity is created inside the combustion chamber using swirlers and recirculation. It helps that the  fuel is burnt with only a portion of the air entering the combustion chamber.

Image from aeromodelbasic.blogspot.in
Basically, the airflow entering the chamber is divided into multiple parts, which enter the chamber at different times and places, so that the overall airflow has low axial velocity, while promoting recirculation. From Combustion process:

Approximately 20 per cent of the air mass flow is taken in by the snout or entry section. Immediately downstream of the snout are swirl vanes and a perforated flare, through which air passes into the primary combustion zone. The swirling air induces a flow upstream of the centre of the flame tube and promotes the desired recirculation. ...
Through the wall of the flame tube body, adjacent to the combustion zone, are a selected number of secondary holes through which a further 20 per cent of the main flow of air passes into the primary zone. The air from the swirl vanes and that from the secondary air holes interacts and creates a region of low velocity recirculation. This takes the form of a toroidal vortex, similar to a smoke ring, which has the effect of stabilizing and anchoring the flame.

Combustors usually feature recirculation zones in which the combustion takes place either completely or which at least anchor the flame. This can be done by several designs

Sudden jump in cross-section in combination with vorticity
Flame-holder bodies
Central displacers
Vorticity brake-down

The blade channel of a compressor is actually divergent. Deceleration of a flow goes along with an increase of the static pressure (see diffusors). Additionally, a compressor transfers work into the fluid, thus further increasing the fluid density. The divergent cross-section of the annular channel between hub and casing is designed to keep the axial velocity somehow level. High axial (or more correctly meridional) velocity levels cause higher friction losses. Low velocities reduce the transferable power of a compressor stage, thus requiring more stages.

While I agree with your answer and up voted it. It would be better to add some links to backup your information.

"The blade channel of a compressor is actually divergent". This is something that in itself should be explained, as the channel is seemingly convergent before the diffuser, each stage of the compressor having a larger channel cross section forward than rearward.

@mins: The annular space between hub and casing differs from the blade (or vane) channel that is defined as the passage between two blades (vanes) of the same stage of a compressor/turbine. You can estimate the flow cross section by zooming into the image attached to the answer and regard compressor/turbine at shaft height. You can see that the channel in a compressor is (slightly) divergent whereas the turbine channel is drastically convergent.

How engineers are able to slow air without decompressing it

Flow speed is energy, so slowing down the flow will increase its pressure.
At subsonic speeds the first part happens already ahead of the intake when the airplane approaches. This pre-compression is very efficient because it happens in the free stream, and the engineers design the intake such that it will only swallow part of the air flowing towards it at cruise speed. Within the intake the flow decelerates further such that it will enter the first compressor stage at a flow speed of Mach 0.4 to 0.5. Typically, 98% of the flow's kinetic energy can be converted into pressure this way.

In the compressor section, air from the intake is forced to rush into
  a convergent annular tunnel to compress it. Intuitively, this design
  won't slow air, or not slow it enough.

Within the compressor the density of the compressed air rises together with the pressure. Therefore, the converging geometry within the compressor does not accelerate the flow - it is actually slowed down further - and only follows the changing volume. The density and volume ratio at isentropic compression are proportional to the pressure ratio (index 0 denoting the initial and index 1 denoting the final state) like this:
$$\frac{p_1}{p_0} = \left(\frac{\rho_1}{\rho_0}\right)^\kappa = \left(\frac{V_0}{V_1}\right)^\kappa$$
with $\kappa$ being the ratio of specific heats (approximately 1.4 for air). The 8.3 compression ratio of the J-85 in your picture will compress the air to 22% of its initial volume.
The final step in the deceleration of the flow happens in the cross section leading from the compressor to the combustion chamber, which is called the diffusor. Here the cross section carefully widens to slow down the airflow without separation. Around the fuel injectors you will find the lowest gas speed in the whole engine. Please see this answer and this answer for more details - I am told not to copy relevant parts of older answers into new answers.

+1, but I'm confused why you voted to close this question as being a duplicate of another question you also answered, and still add a new answer here. Do you think your first answer is not good enough? In that case why not just update it instead of posting a new one?

@mins: No, I felt that aeroalias' answer is incomplete and that this question needs an adequate answer. The answers to the other questions should be adequate, only your suspicion about the converging cross section along the compressor needed to be addressed here in detail. However, anyone else looking for flow speed information should be adequately served with the other answers, hence the close vote.


text:
What is this aircraft with wings seemingly fixed the wrong way?
In this page there is this image:

Is it a concept model or a real model?
If this is the latter, then are there real aircraft with such wings?

What do you mean by wings fixed the wrong way?

@aeroalias they appear to be installed backwards. Usually the leading edge of the wing makes an angle with the fuselage (sweep angle), and the trailing edge is more perpendicular.

@aeroalias: Compared to the Mustang. Here the leading edge seems to be at right angle with the fuselage, but after googling around this seems not so rare.

Note that plenty of aircraft even have forward-swept leading edges. For example Ka-13 glider had this to allow flexibility in balance.

Most of the stuff made by Mooney has a similar wing profile

It's now a collector's item, like this U.S. stamp with an upside down Jenny.

@mins, by wrong,way, do you mean forward sweep wing?

@ManuH: Wrong = The leading edge seems to be at right angle with the fuselage.

@Andy : indeed. And although leading edges with a smaller angle than trailing edges are extremely rare in larger aircraft, they are very common in case of light general aviation aircraft and gliders.

It looks like the Cessna Citation II of which hundreds were built.

By Adrian Pingstone - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2971403
The wing profile is trapezoidal, as can be seen in the image below.

Image from avistar.org
This kind of wing design is found in a number of aircraft, for example the other Cessna Citation series aircraft and the Embraer Phenom 100.

By Josh Beasley from USA - Private - Embraer Phenom 100 - N629AS (26/365)Uploaded by tm, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=25761759

How deceptive is the image I posted regarding the angles! (and Wikipedia's even more!) Thanks for providing this very useful answer.


text:
Is medical evacuation aircraft D-AIGZ "Robert Koch" protected against decompression?
In 2014, Lufthansa engineers converted a A340 into a medical evacuation aircraft specially equipped to carry contagious patients, e.g. with Ebola fever (D-AIGZ, "Robert Koch", msn 347). This aircraft was at the time the property of the German Government (as of today, 2020, it is back to Lufthansa).

Source.
The patient is in an isolation unit surrounded by an airtight tent with negative pressure:

Source.
I was wondering about how this isolation system would react in case of in-flight decompression and how the cruise altitude would be selected. Is it common to have negative pressure tents in civil medical aircraft? Is it safe? Is it regulated?
More:

Inside the flying quarantine used to transport Ebola patients, on Wired
It seems this airplane has been recently converted back to commercial transport.

Is in-flight decompression common?  No, it is extremely rare.

Are you asking about regulations from a foreign standpoint (the specific country that D-AIGZ is registered in), or from a different regulatory body?

@RonBeyer: Not for a specific country. Maybe a regulator has looked at this aspect more deeply than other, that would be fine too.

I'll try to answer in a general sense. It appears that the Lufthansa engineers have changed only the interiors- basically they've changed the passenger fittings with the containment unit. As such, the effect of decompression will be the same as in commercial aircraft- in case of negative pressure containment unit, the air will tend to leak from it. In general, there are some safeguards in such cases:

By sucking air out through the filters, a negative pressure is created within the unit, so that while fresh air may leak in, contaminated air will not leak out. An alarm sounds if negative pressure is lost.
...  in case the negative pressure protection failed, the air inlet at the front (crew-facing) side of the tent is also fitted with a filter to prevent airborne contamination of the main cabin. As a further protection for the crew, Phoenix Air’s mechanics have reversed the airflow in the whole aircraft, so that now it flows fore to aft (instead of aft to fore as it comes from the factory); the pilots and medical crew are in effect located ‘upwind’ of the patient.

However, as @abelenky already noted, in flight decompression is quite rare and the people on board have to worry more about the patient they are transporting than decompression.
Negative Pressure tents with HEPA filters are usually used only in medivac cases where the diseases are highly infectious. In other civil medivacs, the patients are usually exposed to the cabin.
There is a FAA approved Aeromedical Biological Containment System (ABCS) devised by CDC and used in US, which uses the same negative pressure principle- so I'd say that it is safe to operate.
For medical aircraft, the cruise altitude is usually set lower- FL200 or so to enable having the cabin pressure altitude at a lower level (for military aircraft, it is maintained at 4000', though it can vary depending on the circumstances). One thing to note is that most medivac aircraft are business jets like Gulfstream and Learjet, which can maintain sea level cabin pressure upto FL200.
Also, any interior modification will have to be approved by the corresponding regulatory body (here its EASA, though I'm not able to find anything about this particular aircraft). Other manufacturers have obtained EASA certification for negative pressure tents and as already seen, FAA certification too.
European Air Ambulance explains the EASA certification process for medical aircraft, which basically involves:

Type certificate for the original aircraft

Supplement Type Certificate for the medical configuration and

Air Operator Certificate for the aircraft equipped with the approved medical configuration.

Image from European Air Ambulance


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Does flying slower actually save fuel?
Following a discussion on travel.SE, I wonder: would flying more slowly actually save fuel?
I suppose that it would save fuel on a volume of fuel per flight hour basis, but the flight would end up being longer, so would the total flight actually save fuel?
I’m under the impression that there is actually an optimal air speed, and diverging from that in either direction would actually use more fuel.
Which one is true?

I don't know about flying but driving slower does save fuel. At 120kph my car runs at 6.5L/100km but at 100kph it's 5.5L/100km. If I'm not in a hurry, I'd rather save 1L of gas instead of saving 10min of time.

@user3528438 Your fuel efficiency may be better at 100kph compared to 120kph, but there still is an optimum. According to Wikipedia that should be between 56kph and 80kph for an internal combustion car. Below that, your efficiency will get worse again.

If fuel consumption would be proportional (i.e.: linear) to speed, then you would not save any fuel, as suspected.

Related: Fuel economy in aircraft. Actually SAS tried to fly at reduced speed, but only for 2 years.

@user3528438: One way driving is different is that your tires still hold up the weight of the car efficiently at any speed, unlike wings that need speed to make lift, with a high angle of attack needed at low speed.  (i.e. Induced drag is 0 for cars.)  As Gerry's answer mentions, planes have a non-zero "max endurance speed" that maximizes the time you can stay in the air, and going lower actually uses more fuel per time, not just per distance. (vs. a car, you can crawl along at idle, that's your max drive time unless you count turning off the car.)

@user3528438 the numbers have to add up - at €2 per liter, I'd gladly pay €12 for an hour of my life at any time.

They already do compared to my car doing 80mph when the best thing to do is 45. A 747's max speed is 20mph over what it's 'econ' is : "econ. 907 km/h (490 kn; 564 mph), max. 939 km/h (507 kn; 583 mph)" - that's half of my error range, but at ten fold to start. They ain't messing around.

As a car goes faster and faster, it needs more and more energy to overcome wind resistance.  I assume it would be similar for airplanes.

@Haukinger For long trips I'll do the same. But for short trips and fragmented time savings, I'll take the money instead.

"but the flight would end up being longer, so would the total flight actually save fuel"  Energy expended is force multiplied by distance.  The duration of the flight is not directly relevant.

For any craft travelling in or on any medium there is an optimum speed. Travelling at any other speed - higher or lower - will use more fuel.

Yes it saves fuel, unless you get really slow (well below cruise speed - more later.)
Since drag (and thus power required) increases with the square of velocity it will save fuel for the total flight. A simple example even if not truly realistic: going twice as fast generates four times the drag and thus four times the fuel burn (ignoring small changes in engine efficiency) and it gets you there in half the time, so it would still cost twice the fuel burn for the flight. And as a side note, drag is proportional to air density. So flying higher reduces drag and fuel burn for the same speed.
You are correct. There is an optimal air speed. It's called Max Endurance speed. It is at the point where total drag (induced and parasitic) is at a minimum. It is significantly slower than normal cruise speeds. Going slower than that will increase power required and fuel burn even as you go slower. This is 'operating on the backside of the power curve.'
But for maximum range, that occurs slightly higher than minimum power speed at the speed for best L/D. Going slower than the max range speed will result in more fuel over a fixed distance.
One caveat is that if you have a head wind or tail wind, the fuel burn for a given distance will change.
And to make things more interesting; the airlines look at total costs for the flight, not just fuel burn. Crew costs are also significant. In the performance computations within the FMS there is an input called 'Cost Index.' Each airline calculates the index (offline) and provides it to the crew for input. It basically provides an adjustment factor based on the relative costs of fuel vs. crew. With high relative fuel costs, the Cost Index will calculate a slower speed to save fuel. If fuel is cheap relative to the crew, the CI will calculate a higher speed to reduce the crew costs (since they get paid by the hour.)

Not only crew costs but maintenance costs (things that have to be done after x flight hours) and capital costs (getting an extra flight per day out of their $100 million airplane).

Parasitic drag power is (to a first approximation) the cube of airspeed, but drag force and parasitic energy per distance is the square of airspeed. I think induced drag power is proportional to 1/speed (cost per distance 1/speed^2).
For ground vehicles you have parasitic drag, dry friction (power ~ speed, energy/distance constant), but no induced drag. On a bicycle air friction is much more of a limit than dry friction.

Not just airlines, but other operators as well - cargo companies and civil surveillance (e.g. powerline/pipeline monitoring or aerial photography) also care about total flight cost (perhaps also constrained by visual/weather conditions for survey flights).

A slower aircraft can carry less passengers a day. At constant number of passengers, more aircraft are required, and overall more fuel is likely to be burnt. The cost index, which takes into account commercial aspects and possibly indirectly this one, might also be a good indicator for environment protection.

@mins While I see your argument, the swing in block times based on cost index isn't enough to get an extra flight per day out of the a/c. And it would be contingent on the CI not changing. Realistically, the CI may swing the block times by 5-10%. The schedules often have more pad than that to cover weather or other delays. If there is passenger demand the airlines will make the flight available. If they can't add a flight due to airport limits, they can change aircraft to one with more capacity.

@Gerry It's not a simple linear matter. Flights need to arrive at a hub with a minimum time to allow connections. Airports have curfews. For these cases, a matter of minutes can make a flight possible. Demand is also not flat; people dislike early morning or late night flights, so shaving 15 minutes off the 5 previous flights changes a lower-demand 8 PM flight into a just-off-work 6:45 flight.

What kind of aircraft are we talking about here? I'm assuming from OP's travel.SE link that OP is interested in an airliner (link is about a 787.) I was under the impression that the range of possible speeds for an airliner at cruise altitude was actually quite narrow and that descending from that would almost always burn more fuel. Which would leave a quite narrow range from normal cruise speed to maximum range speed (which is what the most fuel-efficient speed would be for a given route.) Some example typical cruise and maximum range numbers from an actual airliner would improve this answer.

Of course, that's quite different from, say, a Cherokee like what I've flown where we have a rather wide (on a percentage basis anyway) range of speeds that we can fly at our normal (much lower) cruise altitudes. Mach buffet isn't normally a problem in the PA-28...

@reirab It's not the airliner, it's the airline. Check your Cherokee POH, you can see cruise fuel flows differing by a factor of at least 1.5. The thing is airlines care about single percentage point differences, which add up to millions of dollars over the lifetime of the aircraft, so they will only normally fly at the limits of altitude and efficiency.

@user71659 Yes, I know it's a significant difference in the Cherokee. What I was referring to with regard to airliners, though, is that flying much slower than they already cruise would typically require flying lower (higher [true] stall speed due to thinner air at cruise altitude,) which would actually burn more fuel, not less, as far as I know. But then, I don't fly airliners, so perhaps my understanding was incorrect. Either way, some numbers on what's actually possible for fuel flows and true cruise speeds for airliners like OP appeared to be interested in would improve the answer.

There is an optimal speed for whatever you are trying to do -- for example, and optimal speed to cover the most distance per unit of fuel is probably what you are thinking of.
However, if you were flying a reconnaissance aircraft, you might care about staying in the air for as long (time) as possible, and there is an optimal speed for that too.
Most transport aircraft are flown at their best range speed.  We sometimes fly these aircraft a little faster than that -- because people also value their time and it doesn't cost a lot more fuel, but can save significant time.
This is all considering the aircraft fixed.  Another option would be to put ourselves in the role of the aircraft designer.
We could design the aircraft differently -- such that the new aircraft's best range speed was slower than the old aircraft.  Such an aircraft might have better fuel economy than the old aircraft.
However, airlines are highly incentivized to reduce fuel burn.  Better fuel economy is one of the only reasons they will replace a fleet of aircraft with a newer model.  Consequently, modern airliners are highly optimized to reduce fuelburn.  Improving on them is very challenging.

Most transport aircraft are flown at their best range speed. - Gerry's answer disagrees, saying they usually they fly somewhat faster, to save on crew costs and maintenance per flight hours costs, etc. while only burning a bit more fuel than at optimal-range cruise speed.  "Near" would be a better word than "at", since my understanding is that most passenger flights (and probably cargo) could save a bit of gas by flying a bit slower.

Re: aircraft design.  I seem to recall some older transport planes having a somewhat higher typical cruise speed, like maybe MD-80?  But https://en.wikipedia.org/wiki/Boeing_737 shows newer 737 models having a higher cruise speed than older 737, now Mach 0.785, up from 0.745.  And Boeing 777 has a Mach 0.84 typical cruise speed, according to google.  If you're willing to sacrifice speed, you can use larger wings, but that's a problem at airports.  (Or your plane has to be smaller.)

Mentour Pilot made a video about some of these airport constraints for increasing the aspect-ratio of a wing while keeping the plane's capacity the same. (wing area has to stay approximately constant) https://youtu.be/zkrdIw8LWrM?si=JS-hJgPOH8OVL6dE&t=603

@PeterCordes I think the highest design cruise speed for a bit transport was the 747.  Newer aircraft have been designed to slightly slower speeds to save fuel.  Business jets on the other hand are playing a game of one-upsmanship, each pushing a tiny bit faster than the last aircraft so their rich owners can have bragging rights.

@PeterCordes I used to tell my students -- all aircraft are span constrained.  If you don't have a span constraint, you just don't know what it is yet.

@RobMcDonald I think the Convair 880/990 was actually faster than the 747, though it seems from some quick searching that those claims were when flying significantly lower than normal airliner cruise altitudes, so Mach number wasn't as high as the 747.

@reirab Could be.  My main point was that cruise Mach for these aircraft peaked in 1960 or 1970 and that aircraft introduced since then have actually been designed to reign that back in.


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Is it possible to find out the actual reason for the diversion of V72578 of 15/04/2022?
There's a weird story making the rounds in the news in France these last few days: Flight V72578 on April 15th was delayed, and then diverted, supposedly because of the destination airport's curfew. Passengers ended up arriving in LUX with a delay of 24 hours. But the facts don't quite seem to match.
The flight was scheduled as NCE-LUX (Nice-Luxembourg), originally departing at 19:35 and arriving at 21:05 (all times local).
After a number of issues, the flight departed over 2 hours late, at 21:40 local time. It flew towards LUX, but when it nearly got there, turned around, and instead diverted to LYS (!):

The official story is that it was getting too late and they couldn't land in LUX because of the airport's curfew.
However:

LUX's curfew starts at 23:00 local time.
There are routinely flights landing very close to that time: LH398 has a scheduled arrival at 22:45 and has recently landed as late as 22:57.
V72578 started turning around at about 22:27, about 100 km from LUX. It would probably have taken 15-20 minutes to land, so well before curfew.
It diverted to LYS (Lyon), 340 km away, at 23:09, which again shows it could have reached LUX in time.
At the time it turned around, it was barely further away from LUX than LH398, which landed at 22:42.

Of course one also wonders why they decided to fly to LYS when they could have diverted to Strasbourg (which is also a Volotea base) which is only 100 km away, much closer to Luxembourg, still open (and actually had a Volotea flight from Nice landing there a bit later that evening) but that's another story.
Is it possible to find out the real reason for the diversion? It seems quite unlikely this was due to the curfew in LUX and it is extremely unlikely it was due to any emergency as it chose to divert to a much further airport. LiveATC does not seem to have coverage for the area, is there any other way to get exchanges with air traffic control which could explain the diversion? Or is there any other source which could be used to determine what really happened?
I'm also surprised this would have happened this late in the flight. It should have been known quite precisely when they would land in LUX shortly after take-off, so shouldn't crew and/or LUX and/or ATC have been aware of any potential issue at that time rather than when beginning descent?
There have been quite a few stories about the social conflict between Volotea and their staff over the last few days. I don't want to think passengers could have been the losing party in some kind of hardball negotiation between crew and management!

I see no reason to think there's any more to it than already stated. Anything else is speculation.

I agree with @GdD. On "It seems quite unlikely this was due to the curfew in LUX" to me it is quite possible. You say at 22:27 they were 15-20 minutes away, but you have no idea what traffic was already in the hold/appoach. There may have been an expected 15-20 minute hold expected (quite usual at large airports towards closing time!).

Was the aircraft scheduled to stay overnight in LUX, or was it needed back in Nice or some other base early morning? Since they would not be able to get the aircraft out of LUX after the curfew, this may be the reason to divert.

I don't know the actual reason, but I suspect that the curfew is indeed it.
If the aircraft would have continued to LUX, it would have arrived probably just before the curfew. Whilst this would get the passengers to their destination, the return flight to Nice (V72579) would still have to be cancelled because it would be unable to depart before the curfew.
It would also mean that the aircraft would have to stay overnight in LUX, and not be available in Nice next morning, or if it was scheduled for another flight that evening, another base. This would result in at least one additional flight being cancelled next morning, and possibly another whole day of knock on effects in the schedule.
It seems that instead they sacrificed the flight to LUX and diverted to LYS. Then, during the night they repositioned the aircraft to Marseille (MRS), from where it performed the early morning service to Dubrovnik (DBR).

Ah indeed... Somehow I had checked the next flights but FR24 displays stuff in AM/PM format in my browser, and I had missed the "AM" in "12:34 AM". Thought it weird it could have stayed in LYS that long! Still interesting that they only realised the issue while they were about to begin descent into LUX.

@jcaron It is probably the Airline's Operations Centre that didn't see the problem in time. Given that Volotea does not have a base in LUX, and that they have only one flight a day there which is scheduled to depart the airport well before the curfew, they probably didn't realise the problem they were about to face until they filed the flight plan for the return flight. In a typical airline operation, the flight crew just flies the aircraft where the companies tells them to fly. The logistics are handled centrally op the OPS centre.


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Is it currently feasible to glide airliners from cruise level to ILS glideslope (CDO)?
Is this possible for some airports to perform full continuous descent operations (CDO), gliding at idle thrust from top of descent to glideslope interception.

(source: Lubo Mitev's Blog)
LAX and UK airports seem to allow some CDO, maybe not from ToD.
What is the current status of such experimentations?
Is this something that could/should be extended to a lot of (or all) commercial planes?

Yes it is feasible, in fact it is no problem at all for an aircraft to perform a Continuous Descent Approach (CDA). The difficulty is that Air Traffic Control (ATC) can not predict accurately how the progress of the aircraft will be during the CDA, so they will need to add extra spacing from other aircraft to ensure safe separation. This reduces the capacity of the airport. Therefore CDA's are only used when there is no need for high runway throughput.
For example, Amsterdam airport uses CDA's from Top of Descent during the night hours and in the early morning. Around 6.30 AM local time they can no longer offer CDA's without risking separation conflicts with other aircraft as traffic start to increase rapidly around that time. They will need to vector and put aircraft on doglegs to ensure separation minima are honoured.


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What happens after a clearance or an instruction has been sent by ATC using CPDLC?
With conventional ATC, an instruction or a clearance is broadcast using voice, in case of no response from the crew, it can be repeated, and if silence persists it can be discarded as if it was never transmitted by ATC.
For aircraft with appropriate equipment, ATC can use CPDLC (controller-pilot data link communication). The message is sent and can be displayed by the crew. Then the crew may accept it or reject it, which is equivalent to the voice protocol.

Source
However what happens in case:

The crew doesn't open the message.
The crew doesn't answer the message after opening it.

Will ATC use voice to ask the crew to expedite responding to CPDLC instruction?
For ATC, how long this message remains valid? Can a message already sent be cancelled?

Voice communication and CPDLC co-exist as means of ATS communication. CPDLC is considered a supplement to voice communication - even if that may change in the future.
CPDLC is only used for non-time-critical communications. This aspect is very important in the context of your question. A response to a message sent via CPDLC can be up to several minutes, whereas for a message sent via voice radio, a reply can be expected within seconds; pilots and controllers know this, and will only use CPDLC if a relatively long response time is acceptable.
Generally speaking, when communication is initiated via CPDLC, the response should be via CPDLC, and when communication is initiated via voice, the response should be via voice. However, there are situations where it is beneficial to revert to voice communication after a CPDLC message has been sent. One example is when it is necessary to ensure the timely execution of an instruction issued by CPDLC, another is when a system generates a time-out or error. There is no standard time limit for replying to a CPDLC message, but various systems probably have some sort of time limits in them.
Standard phraseology is established for situations where it is required to revert to voice communication. One example is:

SAS123, disregard CPDLC climb message, break, climb FL330

In this case a CPDLC message to climb sent to SAS123 is no longer valid, and instead the crew should react to the voice instruction to climb to FL330.
If the sender of a CPDLC messages wants to cancel a previous message, this is indeed possible using CPDLC - but most of the time it is probably desirable to do so via voice, to make sure the message is conveyed immediately. The ICAO Standard CPDLC message for cancelling a previous message is UM168:

DISREGARD

Answer largely based on Introduction to CPDLC Operations (SKYbrary) and PANS-ATM Appendix 5.


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How much air is recirculated (vs. bleed air injected) in modern airliners cabin?
Air in the cabin of airliners is a mix between:

new air (engine bleed air or ram air in some aircraft) and
recycled air pumped out of the cabin, filtered and re-injected.

How much air is recirculated? What are the reasons for recycling air instead of using bleed air only?

In modern aircraft cabin, close to half of the cabin air is recircualted. As this IATA briefing paper on Cabin air quality - risk of contagious viruses notes:

The cabin air system is designed to operate most efficiently by delivering approximately 50 percent outside air and 50 percent filtered, recirculated air.

The AIVC also notes in Air quality in passenger aircraft:

In early commercial jet aircraft, passenger cabins were ventilated with  100% outside air. In more recent jet aircraft, approximately 50% of the  ventilation air is outside air and the remaining 50% is filtered recirculated cabinair.

The reason for using recirculated air is quite simple- fuel savings. From the document Commercial Airliner Environmental Control System- Engineering Aspects of Cabin Air Quality:

At the beginning of the commercial jet airliner age, jet airplanes did not have cabin air recirculation systems,... The primary reason was that early jets were powered by highly inefficient turbojet engines.
As engine technology progressed, turbofans were developed with a core bypass ratio of approximately 2 to 1. Fuel economy improved and the cost of
  engine bleed air relative to overall fuel consumption was still sufficiently small to make 100% bleed air to the passenger cabin cost effective.
As modern turbofan engines with high 5 to 1 bypass ratios were developed, fuel consumption to provide engine thrust decreased. However, the fuel consumption relative to extracting bleed air dramatically increased, almost in direct proportion to the higher bypass ratio. For a 767 with P&W 4000 engines, the percent increase in fuel consumption due to bleed air only would be almost four times higher than an equivalently sized turbojet for the same amount of bleed air.

Increase in bleed air fuel consumption with modern jet engines; image from Commercial Airliner Environmental Control System- Engineering Aspects of Cabin Air Quality by Elwood H. Hun et. al.

From an own work report, based on public web presentations for type rating and on The Boeing 737 Technical Guide by Chris Brady:

A recirculation system reduces bleed air requirements and pack loads, by filtering cabin air and re-introducing it to the mix manifold. Approximately 25% of the cabin air is recirculated. The recirculation fan will switch off if either pack is in HIGH flow, causing a net reduction in ventilation rate of about 15%. Total ventilation rate on a 737-800 is approx 1 m3/sec

A portion of the conditioned air from the left pack flows directly into the flight deck. The remainder of the left pack air, all of the right pack air, and air from the recirculation system is combined into the mix manifold. Mix manifold air is distributed to the passenger cabin through different zones, via the side-wall risers on the left and right sides of the aircraft. On ground, pre-conditioned air can be pumped directly into the mix manifold

The B737 Technical Guide further mentions that on the B757 and B767, up to 50% of cabin air is recirculated, and on the MD80 none.


text:
What is the strongest jetstream ever encountered in commercial flight?
Jet-streams are high-altitude winds which flow from west to east. They can be provide a very strong tailwind or headwind depending on which direction the aircraft is going.
What is the historic record of the strongest jet-stream ever countered by commercial aircraft? I'd expect the result to be either a very short east-going flight (much shorter than the expected time of travel), or a very long west-going flight.

Jetstreams generally don't stay perfectly straight. They tend to follow a meandering, curved path so it would be rare for a flight to stay in the max tailwind for very long.
Dispatchers will often take advantage of jetstreams and plan the routing to follow part of the jetstream core.
There is a website called Ground Speed Records where pilots can post photos of high groundspeeds for an "Unofficial" record.
Here is an example from that website giving a groundspeed of 736 knots from a 240 knot jetstream tailwind. That is probably about the highest jetstream speed you will see:  B747-400 Ground Speed Records

This is an answer to I'd expect the result to be either a very short east-going flight (much shorter than the expected time of travel), or a very long west-going flight.
Air India Flight 173 is probably the best example for this. The flight now travels over Pacific Ocean, enroute from New Delhi to San Francisco. This is opposite to flying over great circle route, but inspite of that Air India reduced the flying time by 2 hours because of jetstreams. Now, the total distance travelled could be anything from 9,300-10,000 miles, and the flight covers it in less than 15 hours.
On October 21, AI 173 flew 10,080 miles in 15.25 hours. Flightaware.
Here is a small video on AI 173, that I have made.

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

@RalphJ but this answer is 203 characters longer than maximum limit for comments. Sorry, I can't comment therefore.